Despite the integration of renewable energy into the public electric energy system (power grid) a large share of electricity is nowadays still generated by fossil energy sources. But the global climate change requires the further development of renewable energies.
The energy output of renewable energy sources like wind and solar is not constant throughout a day or throughout a year. Consequently, electricity which is generated by utilizing energy from renewable energy sources fluctuates.
In order to handle this fluctuating electricity energy storage units are developed. Such energy storage units are a) mechanical storage units e.g. pumped hydro storage, compressed air storage or flywheels, (b) chemical energy storage units e.g. storage of hydrogen, batteries and organic molecular storage, (c) magnetic energy storage units, and (d) thermal energy storage units with water or molten salts.
However, only pumped hydro storage is already today well-established and matured as a large scale energy storage technology. All other storage technologies are lacking capability to store electric energy at low cost, whereas pumped hydro storage is geographically limited to certain regions (sufficient geodetic heights).